The present invention relates generally to methods and apparatus for rewarming frozen blood and tissue, and more particularly to a combination microwave and forced air apparatus for rapidly, and without damage, rewarming frozen blood and tissue.
Without a safe method for long term storage and later restoration of blood and organs, people will die. Without a fast and high capacity method for restoring stored blood and organs, people may die. The need for rapid rewarming of frozen blood is particularly acute in battlefield hospitals.
Donated blood and organ tissue are generally stored either frozen or refrigerated to maximize safe storage life. They must be thawed and/or rewarmed to normal body temperature before transfusing or transplanting into a patient. Transfusing or transplanting inadequately rewarmed blood or tissue into a patient can cause hypothermia, cardiac arhythmias and coagulopathy.
A number of methods, and implementing apparatus, have been developed in the past to safely rewarm blood and tissue. A useful discussion of that prior art, some of which is repeated or paraphrased in this Background of the Invention, may be found in U.S. Pat. No. 4,801,777 to Auerbach. The Auerbach patent is for a combination microwave and warm water blood rewarming method and apparatus intended to avoid many of the problems of the other prior art microwave or combination microwave and warm water methods.
The most common method for rewarming blood and tissue, particularly for rewarming refrigerated whole blood, is a standard water bath method in which blood is passed through coiled tubing immersed in a temperature-controlled water bath. The problems and difficulties of this unwieldy method are well known in the art and are discussed in the Auerbach patent.
Frozen blood or plasma presents an additional difficulty in that the blood or plasma must first be thawed before it can be rewarmed. The most common method for thawing has been simply to place the plastic blood bags containing the frozen blood or plasma into a warm water bath to thaw, followed by conventional rewarming methods.
Unfortunately, the conventional prior art methods and apparatus for rewarming blood and tissue are generally both slow and can rewarm only small volumes at a time.
Microwave and radiofrequency induction heating have been explored as methods for rewarming blood both faster and in greater volumes. These methods have nob worked well, however, for a variety of reasons. The primary problem has been differential excessive heating, or spot heating, which causes cell damage to whole blood cells. Mechanical agitation and rotation have not solved this problem. A particular problem is that blood inside narrow diameter tubing, an integral part of blood storage bags, frequently overheats and even explodes under microwave heating. Careful monitoring and timing of intermittent microwave energy exposure also has not worked successfully for rewarming whole blood because, among other reasons, of the difficulty of effectively monitoring temperature and time.
Microwave heating has been used slightly more successfully to thaw frozen plasma, which does not have the more prone to damage blood cells of whole blood and packed red cells. Even here, however, the method is complex and unwieldy. The frozen plasma is first softened by placing the bag under warm running water, followed by sealing and removal of all tubing to avoid bursting. The bag is then dried and placed inside a plastic overbag to protect all metal clips and remaining narrow tubulature. Microwave exposure is applied in short (10 second) increments under constant observation until the thaw is completed. At this point, the plasma is only thawed and still must be rewarmed to body temperature.
As mentioned, the primary problem of using microwave heating for thawing and for rewarming blood and tissue is the development of hot spots that leads to cell damage. This is particularly true when dealing with microwave or radiofrequency radiation of frozen material because frozen material is a poor absorber of those energies, increasing the risk of hot spots at the first spot volumes to partially thaw.
Thus it is seen that there is a need for a faster and higher capacity method and apparatus for rewarming blood and tissue, and particularly for thawing and then rewarming frozen blood and tissue.
It is, therefore, a principal object of the present invention to provide an apparatus and method for thawing and rewarming blood and tissue that takes advantage of the speed and capacity advantages offered by microwave radiation heating.
It is another object of the present invention to provide microwave radiation heating of frozen blood and tissue without the usual localized overheating or underheating associated with microwave heating.
It is a feature of the present invention that it uses forced air instead of a water bath to remove and distribute excess heat.
It is another feature of the present invention that its microwave radiation is more continuous, or less intermittent, than prior art systems and is thus faster.
It is an advantage of the present invention that its steady-state microwave irradiation is less injurious to red blood cells.
It is another advantage of the present invention that its forced air system is less bulky than water bath systems.
It is also an advantage of the present invention that it is straightforward and easy to use.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.